The present invention relates to a semiconductor apparatus and a technique for manufacturing the same, and particularly relates to a technique effective in application to Pb free solder for fixing a semiconductor device.
A background-art alloy member for die bonding uses a pure Al plate or a 42 alloy material 0.05-0.5 mm thick as a substrate, and an alloy joint layer 0.005-0.1 mm thick is provided on each surface of the substrate.
Further, a Pb free alloy having a liquidus temperature not higher than 400° C. and a solidus temperature not lower than 280° C. or a Pb free alloy having a liquid-phase volume ratio not higher than 15% at 280° C. is used as the alloy joint layer (e.g. JP-A-2001-127076 (pp. 2-3)).
The following four items are important to properties of a solder material applicable to die bonding for a semiconductor apparatus. The first item is a condition as to a process temperature. To make it possible to perform die bonding at 400° C. or lower, the liquidus temperature must be not higher than 400° C., and the wettability with a lead frame material must be excellent. The second item is a condition that the solder material must have heat resistance high enough to be proof against heating and cooling at 260° C. when the semiconductor apparatus is secondarily assembled. Thus, the solidus temperature must be not lower than 270° C. The third item is a condition that when an Si chip is joined to a die pad of a Cu alloy, a solder joint layer must relax thermal strain caused by a difference in thermal expansion so as to prevent the Si chip from being damaged due to thermal stress. The fourth items is a condition that the thermal fatigue life of the solder joint layer must be enough long relatively to a temperature variation caused by heating of the semiconductor device.
A ZnAl based solder material such as a ZnAlGe material or a ZnAlMgGa material known in the background art has solidus and liquidus temperatures at 309° C. and 347° C. or 359° C. and 375° C., satisfying the condition that the process temperature in die bonding must be not higher than 400° C. and the condition that the heat-resistant must reach 260° C. in the secondary assembling process. However, the coefficient of thermal expansion of the solder material is high, and the material is so hard that the elastic modulus or the yield strength is high. Thus, there arises a problem that the chip may be cracked. In addition, the wettability with a lead frame material of Ni or Cu is low due to Al forming a solid oxide film on the solder material. Thus, there is a problem that excellent die bonding cannot be performed.
On the other hand, SnSbAg (15-20 wt %) solder satisfies the condition as to the process temperature not higher than 400° C. in die bonding or the condition as to the die bonding property, and the condition as to prevention of chip cracking due to thermal stress. However, the solidus temperature of the SnSbAg solder is 250° C., lower than the liquidus temperature ranging from 280° C. to 315° C. Therefore, the solder material is partially fused in the assembling process carried out at 260° C., so that an external force to exfoliate the chip by cracking is given to the chip due to thermal expansion of resin serving as an encapsulant at that time. Thus, there is a problem that the chip exfoliates by cracking at the solder joint layer. Further, in a temperature cycle test where a temperature change caused by heating of the semiconductor device is simulated, there is a problem on the temperature cycle reliability because exfoliation by cracking due to fatigue occurs in the SnSb solder in a short time so that the electric properties of the device deteriorate suddenly. SnSb (20-40 wt %) solder has problems as follows. That is, the solder is so hard that chip cracking occurs. The solidus temperature of the solder is 250° C. so that the solder cannot sustain reflow at 260° C. SnSb (not lower than 43 wt %) has a problem as follows. That is, the liquidus reaches 400° C. or higher so that die bonding is difficult at 400° C. or lower.
JP-A-2001-127076 discloses jointing alloys such as ZnAl based alloys, ZnSn based alloys, AuSn based alloys, AuGe based alloys, etc. However, any Zn based alloy has a problem that its wettability is low, and any Au based alloy has a problem that its cost is high.